Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p51e1170d&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P51E-1170
Other
[5475] Planetary Sciences: Solid Surface Planets / Tectonics, [6221] Planetary Sciences: Solar System Objects / Europa, [6222] Planetary Sciences: Solar System Objects / Ganymede, [6223] Planetary Sciences: Solar System Objects / Callisto
Scientific paper
Depth-diameter curves for fresh craters on the icy Galilean satellites reveal that during their formation, large craters sense deep ductile ice and underlying oceans. In addition to a Transition I marking the change from simple to complex craters (as on the Moon), Schenk [Nature, 417, 2002] noted Transitions II and III, along with changes in the craters’ morphology, with both transitions occurring at smaller diameters for Europa than for Ganymede and Callisto. While Transition II was interpreted as sensing ductile ice at depth, Transition III likely marks the crater size at which an ocean is sensed, and this transition was used to infer the depths to the oceans. Notably, the ice-water interface for Ganymede and Callisto was estimated to be ~80-105 km deep, shallower than the 150-200 km estimated from magnetometer data and the expectation that the ocean should form near the ice I and ice III interface at 208 MPa. Here, we explore the possibility that topographic relaxation of "fresh" large Transition-II craters may bias the estimate of Transition III to smaller diameters. Fresh large craters in these satellites may be of substantial age, potentially 1 Gyr for Ganymede and Callisto. We extrapolate the "fresh" depth-diameter curve for Transition-II craters on Ganymede past Transition III and use these dimensions as the initial shape in finite-element simulations of topographic relaxation using the current expected background heat flow. Over a simulated time range of 1-100 Myr, these craters on Ganymede are modestly relaxed, not enough to be easily identifiable as such (fairly subtle upbowing of the crater floor) but enough to affect the depth-diameter curve. Thus, the actual Transition III may occur at somewhat larger diameters, and by inference, the ice-water interface may be more in line with the other estimates. We will perform similar analyses for Callisto and Europa, and while we expect the Callisto results to mimic those of Ganymede, we do not expect a significant change in the expected ocean depth on Europa of ~20 km.
Dombard Andrew J.
Lederer A. P.
Schenk Paul
Turtle Elizabeth Pope
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